Traditionally, the notion of "class" has been used as a model of conceptual organization and development. On this view concepts are considered as having an extensional aspect comprised of the class of all instances of the concept and an intensional aspect which specifies the defining properties of the concept. Membership in the class can be determined by evaluating a potential element with re@pect to the intensional criteria. This proposal presents an analysis of a type of concept which is not readily characterized by the class model--the collection. A collection is the referent of a collective noun, e.g., team, family, pile, bunch. Collections are argued to differ from classes in (1) the processes by which membership can be determined, (2) their part-whole relationships, (3) their internal structures, and (4) the nature of the higher order units they form. From this analysis it is hypothesized that the integrity of the whole formed when elements are considered to form a collection is intermediate between that of an object and a class. Parts of an object are organized to form a whole in a literal sense while elements of a class form a single unit in only a more abstract sense. To test this hypothesis, studies were conducted which utilize a modified Piagetian class-inclusion paradigm. Modifications allowed part-whole comparisons to be made for collections as well as classes. These studies supported the hypothesis, in that part-whole comparisons were facilitated for collections over classes. In Experiments I and II we (a) test an alternative linguistic explanation for this finding and (b) determine whether there exists an ordering of the integrity of the whole from classes to collections to objects. Subsequent studies are delineated which introduce new paradigms to study the importance of the class-collection distinction for children's and adult's performance on memory and inferential reasoning tasks. The results of these studies will provide the basis for formulating an alternative characterization of conceptual organization.